Capacitive transducers have previously been provided, which comprise a thin polymer film where a first electrode, in the form of a first electrically conductive layer, is arranged on a first surface of the polymer film, and a second electrode, in the form of a second electrically conductive layer, is arranged on a second, opposite, surface of the polymer film. Thereby the electrodes form a capacitor with the polymer film arranged therein. If a potential difference is applied between the electrodes, the electrodes are attracted to each other, and the polymer film is compressed in a direction perpendicular to the electrodes, and elongated in a direction parallel to the electrodes. If the transducer is designed in a careful manner, this results in a mechanical stroke from the transducer, i.e. the electrical energy supplied to the electrodes is converted into mechanical work, i.e. the transducer acts as an actuator.
Similarly, if the electrodes are mechanically attracted to each other, and if a potential difference is then applied between the electrodes, then by mechanically decreasing the distance between the electrodes, i.e. by compressing the polymer film in a direction perpendicular to the electrodes, and elongating the polymer film in a direction parallel to the electrodes, it is possible to convert mechanical energy into electrical energy. If the transducer is designed in a careful manner, this results in mechanical work being converted into electrical energy, i.e. the transducer acts as an electrical energy generator.
Similarly, if the electrodes are pushed towards each other or pulled away from each other, the capacitance of the capacitor is changed, due to the altered distance between the electrodes. Such a mechanism can be used for sensing purpose, where any dimensional changes in the transducer can be monitored by reading resulting capacitance changes.
It is desirable to be able to manufacture such capacitive transducers in long and thin webs, preferably using suitable manufacturing techniques, including spin coating, spray coating, casting and/or roll-to-roll processes, in order to be able to mass produce the transducers in an easy and cost effective manner. The materials which have previously been selected for the polymer film have a relatively low viscosity, in the region of 1,000-100,000 mPa·s, prior to curing the polymer film, in order to allow easy handling of the polymer material in the processes described above.
When the transducer is operated in such a manner that electrical energy is transformed into mechanical work, or vice versa, the performance of the transducer depends on the strength of the electrical field applied across the polymer film of the transducer in the sense that the higher the electrical field strength achieved, the larger the elongation of the transducer, in the case that the transducer acts as an actuator, or the more electrical energy is gained, in the case that the transducer acts as a generator. However, when high electrical field strengths are applied across the polymer film, there is a risk that electrical breakdown occurs in the transducer, resulting in damage to the electrodes and/or to the polymer film. The ability of the polymer film to resist electrical breakdown is referred to as the electrical or dielectric breakdown strength. This is a material property. However, impurities, imperfections or defects in the material tend to reduce the electrical breakdown strength. When operating the transducer, the applied voltage should be kept sufficiently low to prevent that the electrical field strength applied to the capacitive transducer exceeds the electrical breakdown strength of the polymer film. Since the applied electrical field strength determines the elongation of the transducer, the maximum possible elongation, and thereby the maximum mechanical work which can be delivered by the transducer, acting as an actuator, is limited by the electrical breakdown strength of the polymer film. Similarly, the maximum possible electrical energy gained for a transducer, acting as a generator, is limited by the electrical breakdown strength of the polymer film. Thus, in some instances it may not be possible to fully utilise the maximum potential of the transducer.